this code looks like it was duplicated by....

why the old form here (SIGNAL() / SLOT()) and the new form for connect?

... this code.

Should the first one be removed?
I can't tell because context is missing. Please use arc diff (arcanist) to upload your future commits.

coding style: newline after if(wallet), I'd recommend curly braces too { ... } even for one-line statements, matching the KF5 coding style.

one space too many before ||

is this include useful? there's no other change to the cpp file, so it looks unnecessary

#define is so 1990 :-)

You can make this proper C++ (and more readable) by doing

static const char s_fillableFormElementExtractorJs[] = "(function(){"
    "..."
    "...";

same here (const char [])

This code makes no sense to me. QWebEnginePage isn't a QWidget, you can't ever cast page to a QWidget.

make it const, so that all the map[...] below don't detach.

should probably be removed

duplicated line

QVector would be much better, since WebField is bigger than a pointer, which means QList has to allocate memory (with new) for every item in the list.

QVector here as well

typo, Ein -> En

Why are there virtual methods in this class? Who/what reimplements it?

I don't like "virtual just in case" because it makes refactoring harder, one has to look for subclasses or assume any method can be reimplemented independently, before changing anything.

If the parent object is always a page (as seems to be the case where "new WebEngineWallet" is done), this code is useless... The caller already makes a best effort at determining WId, so it seems to be this whole if (!wid) code can be removed, no?